Human tooth structures (mineral and proteinaceous components) are adversely affected by caries, and the resulting cavities usually do not fully regenerate. In such scenarios, the affected tooth frequently requires reconstruction using adhesive and restorative dental materials. Polymeric dental composites have been widely used in the restoration of tooth decay or cavity that occurs by primary caries or traumatic events. These materials, along with appropriate adhesives, are noted for their efficacy in restoring the function and appearance of tooth structure. However, these dental restorations may fail due to secondary caries, and replacement of failed restorations creates extra pain, anxiety, and economic burdens for the patients. Dental restorations fail for a variety of reasons. For example, magnified stresses (stress singularity) at or near the tooth/composite interface due to the mismatch of tooth and restorative mechanical properties are important contributors to failure. Polymerization shrinkage that occurs during dental composite curing process has been implicated as a major stress source for the interfacial stress singularity. This shrinkage can lead to marginal micro cracks and subsequent micro leakage at or near the composite-tooth interface, which permits bacteria to pass beneath the restoration surface and ultimately cause secondary (recurrent) caries.
Polymerization stresses (PS) of resin-based dental materials have been measured using a variety of methods. In general, the development of PS is measured through perturbing physically constrained specimens, and the resulting PS can be determined from responses of constraints to the perturbation. A cantilever-beam based instrument has been used for determining PS from the constrained polymerization shrinkage. Composite (or resin) specimens are mechanically attached to a cantilever beam via a quartz rod adhesively in contact with the specimen, and the specimen is also adhesively attached to a fixed lower rod. Upon polymerization, the composite shrinkage stress induces a deflection in a calibrated cantilever beam, and the beam deflection is measured using a linear variable differential transformer (LVDT). The PS is calculated from the measured deflection using a mathematical relationship. Such instruments have not been found to be sensitive to subtle differences in PS. For example, beam deflection measured by existing system does not reflect exclusively the specimen deformation due to shrinkage, but includes deformation from the shrinkage-transmitting rods in the system. In the existing system, calculations for deducing polymerization stress do not take into account geometrical conditions of beam. Further, calibration of the existing system needs compressed air and a load cell (extra instrumental setups) and unnecessary experimental procedures, which become a burden for periodic recalibration of the system. Furthermore, the existing system is solely used for the measurement of polymerization stress development, which cannot be functionally related to the extent of polymerization shrinkage or the modulus development.
Consequently, a need exists for a cantilever-based instrument for measuring the development of PS, shrinkage, and modulus with enhanced sensitivity to specimen shrinkage and beam geometry.